by Christoph Streicher, Ph.D. and Stephanie Chambers

Molecule eucalyptol Eucalyptus oil 1,8-cineolMany think of essential oils as embodiments of the spirit of a plant. But from a less esoteric viewpoint they are defined as products obtained from either the distillation of a raw natural material with water or steam, or in the case of citrus rinds, from mechanical processing.

They can be derived from the flowers, leaves, fruits, roots, stem or other parts of a plant. In some cases a plant  produces them in only one part (e.g. Rose petals), but some plants like the bitter orange tree produce a variety of essential oils with the leaves generating Petitgrain, the flowers giving Neroli and the fruit peel - Bitter Orange essential oil. Some plants have more oil glands or ducts and thus produce more essential oils and are less expensive. But only a small percentage of flowering plants produce essential oils of any quantity.

Essential oils can also be viewed as compositions of 50 – 300 chemicals and as such they can react with other chemicals. So what are they in terms of chemicals?

All essential oils consist of:
• hydrocarbons
• oxygenated terpenes.

Being a hydrocarbon means that like all organic matter, it consists only of hydrogen and carbon atoms. The hydrocarbons in essential oils are Monoterpenes and Sesquiterpenes, and rarely Diterpenes. Sesquiterpenes and Monoterpenes are present in almost all essential oils. Examples of essential oils with a high percentage of oxygenated Sesquiterpenes are Sandalwood, Cedarwood, Myrrh, Vetiver and Patchouli. Examples of essential oils with a high content of Monoterpenes are all citrus oils (e.g. Lemon, Grapefuit, Orange) and conifers (e.g. Balsam Fir).

Being an oxygenated compound just means that one (or in rare instance two) oxygen atoms have bound to the Terpene or Sesquiterpene molecules.

These types of oxygenated compounds are found in essential oils:
• Phenols
• Alcohols
• Esters
• Aldehydes
• Ketones (not very common)
• Acids (not common)
• Oxides. 

Which essential oils contain more of these oxygenated compounds?

Examples of essential oils that contain a lot of:
• Phenols – Clove
• Alcohols –  Geranium, Rosewood and Rose Otto
• Esters – Bergamot, Roman Chamomile, Clary Sage and Lavender
• Aldehydes – Lemongrass and Cinnamon Bark
• Ketones – Sage, Hyssop, Rosemary and Western Red Cedar
• Acids – Birch and Wintergreen
• Oxides – Ravintsara, Rosemary 1.8 Cineol Oil and all types of Eucalyptus (the chemical structure of a molecule of eucalyptol also known as 1,8-cineol is shown in the illustration above) except Eucalyptus Citriodora which contains aldehydes.

The above compounds are the chemical constituents of about 99%of all essential oils. They can also contain small amounts of Alkanes, Ethers, Carboxylic Acids, Lactones, Coumarins and Furanoids.

Sometimes the geographic location in which a plant is grown and / or the genetic makeup of the plant can strongly affect the chemical nature of the essential oil it produces. These are known as the chemotypes of the species. For example, Rosemary essential oil has three different chemotypes. So a plant grown in one location can have more camphor, another 1.8 cineole and a third can have significant amounts of  verbenone. Generally the hotter the area where the Rosemary is grown, the more cineol will be in its essential oil. This is true for other cineol rich oils also like peppermint and eucalyptus. In the case of basil, the two different chemotypes, linalol and methyl chavicol are strictly caused by the genetic makeup, even though the linalol type originated in France, and the methyl chavicol type originated in the tropics. That’s why it is still called tropical basil.

The chemotype also affects the therapeutic uses of the oil. That is why you might have noticed that at Amrita we sell three types of Rosemary Essential Oil – Rosemary Borneol (which can oxidize to camphor) which strengthens the immune system, Rosemary 1.8 Cineol which is good for the respiratory organs and dandruff, and Rosemary Verbenone which also strengthens the liver. All three types are also good at clearing the mind. Because Rosemary 1.8 Cineol is the most commonly produced, it is the least expensive. Verbenone is generally considered the finest type of Rosemary essential oil.

What is an example of the constituents of an essential oil?

Grapefruit essential oil, like orange oil, consists mostly limonene and a few other terpenes. But it also contains a ketone called nootkatone which is considered responsible for the euphoric effects of grapefruit.

How can this chemical knowledge be applied?

At Amrita Aromatherapy we use our own gas chromatograph to check that each essential oil we supply is pure and unadulterated. We check that the oils haven’t been diluted with alcohol or any other solvent and that they do not contain so-called “nature identical” laboratory-produced compounds. See a page about our in-house laboratory for more details.

You can learn a lot about an essential oil medical properties by looking at how viscous (how sticky it is) and how volatile it is. And if an essential oil contains a lot of phenols or ketones you have to be very careful in using it as it could be toxic.

Most essential oils can’t be safely used directly on the skin without dilution. However, there is no harm in applying essential oils that are mostly comprised of Sesquiterpene alcohols such as Sandalwood, Vetiver and Patchouli. Because the molecules in these are larger, they are less aggressive.

In an attempt to make things appear simple, many aromatherapy educators try to assign certain therapeutic properties to certain classes of essential oil molecules. For example, they say that esters are very mild, and emotionally uplifting, alcohols are calming, aldehydes are calming, and that ketones stimulating.  This works to some extent, but there are many exceptions. Many alcohols are indeed calming, like linalol,  geraniol, but then there is menthol, which is rather stimulating. Esters are commonly among the least toxic compounds, by then there is sabinene acetate, a rather toxic compound.

Assigning therapeutic properties to essential oil compound classes gets us only so far. This needs to be complemented by a more holistic view of the properties of the oils.

Disclaimer: The statements made in this blog have not been evaluated by the U.S. Food and Drug Administration (FDA). They are not intended to diagnose, cure or prevent any disease. If a condition persists, please contact your physician or healthcare provider. The information provided is not a substitute for a face-to-face consultation with a healthcare provider, and should not be construed as medical advice.